这句还原成陈述句就是this factory is...
那么这句中的factory 显然不能做先行词.而定语从句必须有先行词,所以就要补上一个先行词,这先行词又要指factory,为避免重复这个名词,就用the one 来代替.这句完整的说法是：Is this factory the one (that) some foreign friends visited last Friday?另外,如果是下一种情况就不一样了：
Is this the factory (that)some foreign friends visited last Friday?